This invention relates to soldering, and particularly to an apparatus and to a method for forming soldered joints without the use of a solder flux.
Soldering is an ancient art and, as well known, it is generally desirable to use certain agents, known as "fluxes," in the soldering process. Such fluxes attack and remove oxides initially coating the surfaces. Freshly cleaned surfaces are thereby prepared for contact by the molten solder, whereby better wetting and adhesion of the solder to the surfaces occurs. Also, the molten solder better flows over the surfaces for uniformly spreading the solder over the surfaces being bonded.
There are, however, numerous instances where the use of solder fluxes is impractical. In photonic devices, for example, an exposed surface is present which serves as a mirror for the light to be emitted. During soldering, such exposed surface can be damaged by any exposure to the solder flux. Accordingly, for avoiding such damage, soldering in the manufacture of certain photonic devices, as well as other devices, particularly in the semiconductor device art, is done without the use of solder fluxes.
To this end, great care is taken to prevent contamination of the surfaces being soldered together whereby proper flow and full surface contacting by the molten solder is obtained without the use of a solder flux. While such fluxless soldering is generally possible, the soldering processes are somewhat "delicate" in that great care must be exercised, the processes used are generally relatively slow, and, not infrequently, poor solder joints result.
Analysis of fluxless soldering processes previously used reveals two principal problems.
In one prior art process, for example, a relatively large first part is to be soldered and the part is heated, prior to the application of solder, to a temperature in excess of the melting temperature of the solder to be used. A solder preform is then placed on the preheated part and a second part to be soldered to the first part is placed on the solder preform. Owing to the high temperature of the first part, the solder melts and flows to form the solder joint. However, while the solder immediately begins to melt when it is placed on the preheated part, there is a tendency for oxides to form on the solder. To the extent that such oxides are formed (which oxides are not removed in the absence of a solder flux), proper wetting of the solder with the surfaces is impaired and poor soldering results.
Another problem is that with various combinations of materials, e.g., a gold surface to be soldered using a lead-tin or gold-tin solder, the molten solder reacts with the material of the surface and becomes, rather than a relatively free flowing liquid, a highly viscous, poor flowing paste. Thus, the solder does not properly spread across the surfaces being soldered and defective soldered joints result.